Device for automatic control of the temperature of an electric furnace



P 1, 1958 w. ROOST DEVICE F OR AUTOMATIC CONTROL OF THE TEMPERATURE OFAN ELECTRIC FURNACE Filed Feb. 8, 1955 ,1 3 3* E C I H. F. ;HF FILTER QTc emsamon 5 I? I we 2% f k 9 SOURCE f3 INVERTER E 5 51 MOTOR m DC SOURCE37 I CONSTANT REFERE'A/Cf VOLTAGE GENERATOR INVENTOR WILLEM TROOST AGENT2,82%,23l Patented Apr. 1, 1958 DEVICE FUR AUTOMATIC CONTROL OF THETEMPERATURE @F AN ELECTRIC FURNACE Willem Troost, Emmasiugel, Eindhoven,Netherlands, assigner, by mesne assignments, to North American PhilipsCompany, Inc, New York, N. L, a corporation of Delaware ApplicationFebruary 8, 1955, Serial No. 486,965 Claims priority, applicationNetherlands February 9, 1954 3 Claims. (Cl. 219-20) The inventionrelates to a device for automatic control of the temperature of anelectric furnace, which is fed from an A. C. source or a diiferentsource of energy. Use is made herein of a suitable pick-up, for examplea therrno-element or a resistance thermometer, which is arranged in ornear the furnace and with the aid of which an electric voltage isobtained, which varies with the furnace temperature. This voltage may beused as a control-voltage, by means of which, if necessary via anamplifier, a control-member is controlled; this controlmember increasesthe supply of energy to the furnace if the temperature is too low anddecreases the supply, if the temperature is too high.

Such arrangements are used with high-frequency furnaces which are fed bya current of high frequency produced by a high-frequency generator. Thecontrolmember may, in this case, be constructed in the form of aslidable core of an inductor connected in series with the winding of thefurnace. The high-frequency generator may be fed from thealternating-voltage source having a frequency of 50 cycles per second.Various kinds of control may be carried out. The invention may be usedin devices in which a so-called proportional control is carried out;with this control the displacement of the control-member issubstantially proportional to the extent to which the real temperaturedeviates from the desired temperature value. This applies to a giventemperature range, the so-called proportionality range. A proportionulcontrol-device responds to the value and the sense of the divergence ofthe magnitude to be controlled within a predetermined range. Theproportional control may be combined with an integrating control or adifferentiating control, in which the control-member is additionallyaccelerated proportionally to the divergence or additionally displacedproportionally to the velocity of the variation in the magnitude, or itmay be combined with an integrating and a differentiating control.

If the load of the furnace increases, so that the temperature drops, thesaid control-voltage occurs, the control-member being thus actuated in asense such that it increases the supply of energy to the furnace. Aftera certain amount of time, which depends mainly upon the heat inertia ofthe furnace, a stationary condition is reached, in which the initialtemperature is substantially restored. In the case of a voltagevariation of the supply source or a variation in condition of the sourceof energy, which implies a corresponding variation in the supply ofenergy to the furnace, the same accurs. The controlvoltage does notoccur until due to the initial variation in the supply of energy to thefurnace, the temperature has varied to a sufiicient extent. Theresultant time lag is in many cases a disadvantage. Stabilization of thesupply voltage can, in general, not be carried out,-since the powerabsorbed by the furnace is comparatively high.

The present invention has for its object the provision of a device ofthe aforesaid kind, in which this disadvantage is reduced to a greatextent. The device of the present invention provides not only a morerapid but also a more accurate control in a sense such that thetemperature variations due to variations in operational conditions arereduced.

in the device of the present invention, the control member is alsosubjected to a control-voltage, derived from the supply source voltageor from a condition of the source of energy, and is operative in a sensesuch that the effect of supply source voltage variations or conditionvariations of the source of energy on the temperature of the furnace isat least partly compensated substantially without delay.

The control-voltage derived from the temperature of the furnace isoperative across a control-circuit, which, in general, is connected tothe input circuit of an amplifier, the output circuit of which governsthe controlmember. This is obtained, in general, by means of a motor, orby means of a pneumatic device actuated by a motor. The control-voltagederived from the supply voltage may govern an additional control-member.However, it is more advantageous to introduce this controlvoltagedirectly into the control-circuit, since the need for an additionalcontrol-circuit with governing members is thus avoided.

The control-circuit may, furthermore, comprise an impedance, acrosswhich occurs a voltage varying with the displacement of thecontrol-member from the initial. position; this voltage counteracts thecontrol-voltage derived from the temperature of the furnace. Thus thedisplacement of the control-member, in the case of a particulardeviation of the furnace temperature from the desired value or in thecase of supply voltage variation or condition variation of the source ofenergy, may be such that it is substantially proportional to the saiddeviation or variation respectively. Thus a proportional control isobtained.

The device is preferably such that the second controlvoltage issubstantially equal to zero in the case of a normal supply sourcevoltage of, for example, 220 volts, or a normal condition of the sourceof energy; the value of said second control voltage being substantiallyproportional to the supply source voltage variation or conditionvariation and being operative in the same sense as such variation.

In order that the invention may be readily carried into effect, it willnow be described more fully with reference to the accompanying drawing,in which the figure is a schematic diagram of an embodiment of thedevice of the present invention.

In the figure, reference numeral 1 designates a highfrequency generator,which is fed from an alternatingvoltage supply source. The alternatinghigh-frequency voltage produced serves to heat an element 2 byinductance; this element may, for example, be a graphite crucible, inwhich the material to be heated and to be kept at a constant temperaturemay be housed. The heating current is controlled by means of a seriesinductor 3, the inductance value of which may be adjusted by means of aslidable core 4 of magnetic material. The core 4 constitutes thecontrol-member proper and is driven by a motor 19 in a manner to bedescribed hereinafter.

In or near the element 2 provision is made of a pick-up, for example athermo-element 2.9. The element 29 is connected through a high-frequencyfilter 5, which is intended to keep the measuring apparatus free frominterfering oscillations, to an indicating device or recording device 6.

The device 6 may comprise a known self-compensating circuit. The voltageproduced by the thermo-element 29, or the first'voltage, is, in thiscase, compensated in a compensation circuit by a known variable voltage,or the second voltage, which may for example be derived from apotentiometer. The residual voltage, which is the first residualvoltage, acts, through an amplifier, upon a motor which drives a pointer7. The pointer 7 is coupled with the slidable contact of the saidpotentiometer, so that the motor, and hence also said pointer, standstill as soon as the total voltage appearing across the compensationcircuit is equal to zero; hence the voltage produced by thethermo-element 29 is equal to the voltage derived from the saidpotentiometer. The position of the pointer 7 thus indicates the value ofthe voltage supplied by the thermo-element 29, or the first voltage,which is at the same time a measure of the temperature of the element 2.At the pointer '7 provision may be made of a dial or scale calibrated indegrees. Such a self-compensating circuit has the advantage that astrong source of energy, i. e. a motor, is available for driving thepointer, so that the assembly may be of a rugged construction.

The pointer 7 is coupled mechanically with a sliding contact 8 of apotentiometer 9, which is connected to a stabilized source of directvoltage. The potentiometer 9 comprises a manually adjustable slidingcontact 19, which determines the desired temperature or the secondvoltage. As will be evident hereinafter, the position of the contact 10determines the temperature to which the furnace is adjusted or thedesired temperature. This is the socalled adjusting point. If thefurnace has exactly the desired temperature, the sliding contacts 8 andM are substantially at the same position. The voltage prevailing betweenthe contact positions, or the first residual voltage, is a measure forthe temperature deviation.

A resistor 11 is connected between the contacts 8 and 10. A voltage,which is the first residual voltage and which governs the controlmember, is derived from the resistor 11 by means of a sliding contact12.

It may be temporarily assumed that the elements 23 to 23, of the figure,are not provided. The contact It? is then connected directly to a device17. The contact 12 is connected through a parallel combination of twopotentiometer resistors and 16, which are connected to a stabilizeddirect-voltage source and along which sliding contacts 13 and 14 aredisplaceable, to the input circuit of the device 17. The contacts 13 and14 determine the component and third voltages, respectively. The device17 is an oscillation converter or inverter, which converts the directvoltage operating across the input circuit into an alternating voltage.This alternating voltage is amplified in an amplifier 18, the outputcircuit of which controls the motor 19. The motor 19 and a generator 20coupled mechanically thereto constitute a motor-generator aggregateknown with such control and measuring arrangements. The motor andgenerator are fed from the same alternatingvoltage source (not shown inthe figure) and the alternoting voltage produced by said generator isfed back to the amplifier iii. The generator 20 provides a dampingcontrol of the movement of the control-member 4, which is driven by themotor 19 (as is indicated by the broken line 21). By control of thefeedback voltage supplied to the motor, the adjustment of thecontrolmember 4- may be oscillating, critically damped or overdamped.

The device of the present invention so far described operates asfollows:

Starting from a stationary condition, in which the temperature of thefurnace element 2 has substantially the desired value, the temperaturedrops at a variation of the load, for example an increase in load. Thepointer 7 moves and the contact 8 moves and a voltage occurs across theresistor 11. The voltage across the resistor 11 is the first residualvoltage. The part of this voltage appearing between the contacts 10 and12 is converted in the device 17 into an alternating voltage having anamplitude proportional thereto; said alternating voltage governing theposition of the control-member 4 through 4 the amplifier 18 and themotor 19. The control member 4- is thus displaced in such a sense thatthe inductance of the inductor 3 decreases and the current through theheating winding or element 2 of the furnace increases.

The parallel combination of the resistors 15 and 16, which may becombined to form a single potentiometer having the sliding contacts 13and 14, insures that the displacement of the core 4 is proportional tothe initial temperature variation. The sliding contact 13 occupies afixed position, whereas the sliding contact 14 is coupled mechanicallywith the core 4. Thus, upon a movement of the core 4, a voltage, whichis the third voltage, is introduced into the control-circuit; thisvoltage counteracts the voltage obtained from the resistor 11 to providethe second residual voltage. The motor 19 and the control-member 4 standstill as soon as the two voltages operating across the control-circuitare equal to one another. Thus a so-called proportional control isobtained, the Width of the proportionality range being adjustable bymeans of the contact 12. The contact 13 serves as a manual after-controlof the adjusting point. Provisions may thus be made that the contact 8coincides in position with the contact 10 in each stationary condition.

According to the invention, provision is made of means for obtaining asecond proportional control in accordance with supply source voltagefluctuations. To this end provision is made of a transformer 23, havinga primary winding connected to the alternating-voltage supply and asecondary winding connected to a rectifier 24. The output of therectifier 24 comprises a smoothing element 25. The rectified supplyvoltage, which is the fourth voltage, is compared with a very constantreference voltage, which is the fifth voltage, derived from a constantreference voltage device 27, which is connected in series with an outputpotentiometer 26. A voltage which is the third residual voltage andwhich is substantially proportional to the difference between the outputvoltage of the rectifier and said constant reference voltage, occursacross the potentiometer 26. The adjustment is preferably such that saidrectifier output and reference voltages are substantially equal to oneanother at the normal supply source voltage. In this case no voltage isoperative across the potentiometer 26 and the controldevice operates inthe manner described above. The constant reference voltage device 27 maycomprise any suitable source of constant potential.

At a variation in supply source voltage a voltage occurs across thelower part of the potentiometer 26; this voltage being also operative inthe control-circuit. In consequence thereof the motor 19 is actuatedsubstantially without delay, so that the control-member 4 is displaced.At the same time, the sliding contact 14 is displaced in such a sensethat in the control-circuit the voltage across the lower part of thepotentiometer 26 is counteracted. The motor 19 stands still when thelatter voltage is equal to the voltage prevailing between the points 13and 14. Thus a proportional control is obtained, this is, however, aso-called anticipating control. This control may also be combined with adifferentiating control. The control proper on a constant temperature iscarried out by the part of the arrangement described above. The width ofthe proportionality range of the second control may be adjusted by meansof a sliding contact 28.

By the combination of the present invention, as described, of twocontrol-effects, a large part of the socalled dead time, i. e. the timerequired after a disturbance of the stationary condition to cause thecontrol-member to become operative, is obviated as far as supply voltagevariations are concerned, so that the control is not only more rapid butalso more accurate.

As an alternative the second control-voltage may be derived not onlydirectly from the supply source voltage but also from the output voltageof the high-frequency generator 1. As a further alternative, said secondcontrol voltage may be derived from a direct voltage operative in thegenerator 1; for example, from the anode supply voltage of the tube ortubes of said generator. In the latter case the transformer 23, therectifier 24, and in many cases also the smoothing element 25, may bedispensed with.

The heat energy may be obtained not only from a high-frequency generatorbut also from the supply source. Thus, for example, the furnace may beheated by a heating medium, for example a liquid or vapor, which isheated by energy derived from said supply source. The secondcontrol-voltage may be derived from the temperature of the heatingmedium and hence indirectly from the supply voltage.

As an alternative, the heating energy may be supplied by a sourcedifferent from the supply source such as, for example, by a gascontainer, the pressure of which varies, or by a vapor duct, thetemperature'of which vapor exhibits variations.

With the arrangement of the present invention as described above, afurnace temperature of about 1000 degrees C. may be kept constant withina tolerance of about half a degree C.

While the invention has been described by means of a specific exampleand in a specific embodiment, I do not wish to be limited thereto, forobvious modifications will occur to those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A device for automatic control of the temperature of an electricfurnace comprising a heating element, means for applying electricalenergy to said heating element, regulating means connected between saidheating element and said energy applying means for varying the magnitudeof energy applied to said element thereby varying the temperature ofsaid furnace, means for varying the regulation of said regulating meanspositioned in operative relation therewith, means for producing a firstresidual voltage in accordance with the difference between a voltagedependent upon the magnitude of the temperature of said furnace an avoltage dependent upon a predetermined magnitude of the saidtemperature, means for producing a third voltage, means for varying theamplitude of said third voltage in accordance with the regulation ofsaid regulating means, means for deriving a fourth voltage dependentupon the magnitude of the voltage of said energy applying means, meansfor produc-,

ing a fifth voltage, means for combining said first residual voltagewith said third voltage thereby to produce a second residual voltage,means for combining said second residual voltage with said fourthvoltage and said fifth voltage thereby to produce a control voltage, anelectrically energized adjusting member coupled to the said regulationvarying means, and means for applying said control voltage to saidadjusting member to vary the magnitude of the temperature of saidfurnace whereby variations in the magnitude of the said voltage of saidenergy applying means are substantially compensated for substantiallywithout time lag.

2. A device for automatic control of the temperature of an electricfurnace comprising a heating element, means for applying electricalenergy to said heating element, regulating means connected between saidheating element and said energy applying means for varying the magnitudeof energy applied to said element thereby varying the temperature ofsaid furnace, means for varying the regulation of said regulating meanspositioned in operative relation therewith, means for deriving a firstvoltage dependent upon the magnitude of the temperature of said furnace,means for producing a second voltage dependent upon a predeterminedmagnitude of the said temperature, means for combining the said firstvoltage with the said second voltage thereby to produce a first residualvoltage, means for producing a third voltage, means for varying theamplitude of said third voltage in accordance with the regulation ofsaid regulating means, means for combining said first residual voltagewith said third voltage thereby to produce a second residual voltage,means for deriving a fourth voltage dependent upon the magnitude of thevoltage of said energy applying means, means for producing a fifthvoltage, means for combining said fourth voltage with said fifth voltagethereby to produce a third residual voltage, means for combining saidsecond residual voltage with said third residual voltage thereby toproduce a control voltage, an electrically energized adjusting membercoupled to the said regulation varying means. and means for applyingsaid control voltage to said adjusting member to vary the magnitude ofthe temperature of said furnace whereby variations in the magnitude ofthe said voltage of said energy applying means are substantiallycompensated for substantially Without time lag.

3. A device for automatic control'of the temperature of an electricfurnace comprising a heating element, means for applying electricalenergy to said heating element, regulating means connected between saidheating element and said energy applying means for varying the magnitudeof energy applied to said element thereby varying the temperature ofsaid furnace, means for varying the regulation of said regulating meanscomprising a control member positioned in operative relation there with,means for deriving a first voltage dependent upon the magnitude of thetemperature of said furnace, means for producing a second voltagedependent upon a predetermined magnitude of the said temperature, meansfor combining the said first voltage with the said second voltagethereby to produce a first residual voltage, means for producing acomponent voltage, means for varying the amplitude of said componentvoltage comprising a first potentiometer having a control memberconnected to said first residual voltage producing means, means forproducing a third voltage, means for varying the amplitude of said thirdvoltage comprising a second potentiometer connected in parallel withsaid first potentiometer and having a control member coupled to the saidcontrol member of said regulation varying means, means for combiningsaid first residual voltage with the said variable component voltage andthe said variable third voltage thereby to produce a second residualvoltage, means for deriving a fourth voltage dependent upon themagnitude of the voltage of said energy applying means, means forproducing a fifth voltage, means for combining said fourth voltage withsaid fifth voltage thereby to produce a third residual voltage, saidlast-mentioned means comprising a third potentiometer having a controlmember connected to the control member of said second potentiometer,means for combining said second residual voltage with said thirdresidual voltage thereby to produce a control voltage, an electricallyenergized adjusting member coupled to the said control member of saidregulation varying means, and means for applying said control voltage tosaid adjusting member to vary the magnitude of the temperature of saidfurnace whereby variations in the magnitude of the said voltage of saidenergy applying means are substantially compensated for substantiallywithout time lag.

References Cited in the file of this patent UNITED STATES PATENTS2,474,441 Sparrow June 28, 1945 2,499,964 McRae Mar. 7, 1950 2,651,704Prior Sept. 8, 1953 OTHER REFERENCES Eubank: Precision Thermostat forHigh Temperatures; The Review of Scientific Instruments; vol. 21, No.10, October 1950.

